Liquid chromatography is a separation and analysis technique that utilizes a liquid mobile phase and various types of stationary phases. In classical liquid chromatography, the mobile phase slowly flows through the column via gravity, making it impossible to use stationary phases with small particle sizes (around 100-150 μm). The separated sample is collected in fractions and analyzed separately, making classical liquid chromatography less efficient and slower in terms of analysis speed. However, the development of high-performance stationary phases with particle sizes smaller than 10 μm, as well as the use of high-pressure pumps and automatic record-keeping detectors in the 1960s, led to the development of high-performance liquid chromatography (HPLC), also known as high-pressure liquid chromatography.
There are six commonly used detectors in liquid chromatography
1.Ultraviolet-visible (UV-Vis) Detector
The UV-Vis detector is the most widely used detector in liquid chromatography, accounting for over 80% of applications due to its excellent sensitivity and resistance to temperature, flow rate, wind speed, humidity, and vibration changes. Its high sensitivity can detect up to 10-9g/mL (naphthalene methanol solution), and it can detect using elution mode with good repeatability.
The sensitivity of the UV-Vis detector depends on solvent effects, background absorption, and differential refractive index effects. Different solvents have their own cutoff wavelengths, and solvent quality affects the cutoff wavelength. Solvent quality is also related to UV absorption by impurities, dissolved oxygen, and buffer solutes. Background absorption reduces the linear range, and many solvents produce background absorption, so careful selection is necessary. Differential refractive index effects can cause false UV absorption changes, resulting in quantification errors and inaccurate spectral profiles, especially during gradient application.
2.Photodiode array (PDA) Detector
The PDA detector can collect both UV and visible spectral data while simultaneously obtaining chromatographic data, making it useful for purity verification and confirmation of chromatographic peaks. It can reprocess data at any wavelength and eliminate differential refractive index effects from hardware. PDA detector performance is typically evaluated in terms of chromatographic sensitivity, spectral sensitivity, and spectral resolution.
3.Refractive index (RID) Detector
The RI detector is a commonly used detector in liquid chromatography, and it can be combined with pumps, columns, and injectors to form gel permeation chromatography or high-speed liquid chromatography systems, or used as a stand-alone analytical instrument with an appropriate injection system. It can detect all solutes, including those that cannot be detected with selective detectors, such as high molecular weight compounds, sugars, and aliphatic hydrocarbons. Because different liquids have different refractive indices, this detector has high versatility and can be widely used in chemical, petroleum, pharmaceutical, and food fields.
4.Fluorescence Detector
The fluorescence detector is a commonly used detector in high-performance liquid chromatography. When the chromatographic fraction is irradiated with ultraviolet light, the sample components with fluorescent properties can be detected. Its characteristics include high selectivity, only responding to fluorescent substances, and high sensitivity, with a lowest detection limit of up to 10-12 g/ml, making it suitable for trace analysis of polycyclic aromatic hydrocarbons and various fluorescent substances. It can also be used to detect substances that do not fluoresce but can fluoresce after chemical reaction. For example, in phenol analysis, most phenols do not fluoresce, so they are first processed to become fluorescent substances before analysis.
The fluorescence detector’s filter can be classified as short-pass (allowing all wavelengths below a specific point to pass through), long-pass (allowing all wavelengths above a specific point to pass through), and band-pass (allowing all wavelengths within a specific range to pass through).
5.Electrochemical Detector
The principle of the electrochemical detector is that a current proportional to the concentration of the compound being tested can be generated as the compound is oxidized or reduced. It is generally used in special situations and is mainly used to determine ions with unstable chemical properties, such as ions that are easily oxidized or reduced.
The characteristic of this detector is its very high selectivity, as only electroactive substances that are easily oxidized or reduced can be detected. For example, even when high levels of chloride and sulfate coexist, the detection of other ions is not affected, as these two ions are not detected by the electrochemical detector.
6.Conductivity Detector
All ionized compounds and dissociable compounds in aqueous solutions can conduct electricity. The conductivity detector uses the change in the conductivity of the mobile phase of the liquid chromatography as a quantitative basis. The mobile phase carrying the sample passes through the flow cell, and the blank mobile phase generates a conductivity value. The conductivity value of the sample with the mobile phase added is subtracted from the conductivity of the mobile phase to obtain the conductivity value of the sample, which is proportional to the concentration of the sample being tested.
The conductivity detector uses a conductive solution as the medium, so a buffer solution is suitable as the mobile phase. However, this unavoidably increases the background baseline of the detector. Therefore, in ion chromatography without a suppressor column, most people use a low concentration of organic acids or organic acid salts as the mobile phase to reduce the background baseline.
The characteristic of this detector is its relatively simple structure and low sensitivity, making it unique for detecting ions. In summary, each type of detector has its own characteristics and different applicable ranges. We should choose the appropriate detector based on different usage environments to improve work efficiency.